Display omitted •An acidic polysaccharide (RBAPS) was gained from fermented burdock residue mediated by R. nigricans.•The structural characteristics of RBAPS were clarified.•RBAPS promoted cell viability and phagocytosis of macrophages.•RBAPS induced the macrophages polarization and pro-inflammatory cytokines secretion. An acidic heteropolysaccharide (RBAPS) with a relative molecular weight of 140.9 kDa was extracted from Rhizopus nigricans mediated burdock residue fermentation. Structural analysis indicated that RBAPS was mainly composed of Rha, Ara, Gal, GalA, GlcA, Glc and Xyl in a molar ratio of 14.5: 14.3: 30.1: 31.0: 5.5: 0.8: 3.8. The dominant backbone of RBAPS consisted of α-Rhap, α-GalAp, and α-Galp with the branched chains of β-Galp and α-Araf. In macrophage RAW264.7 cells, RBAPS promoted proliferation, enhanced phagocytosis, hindered apoptosis, and promoted cytokines (NO, IL-6, TNF-α, and IL-1β) secretion. RBAPS also promoted M1 macrophage transformation and induced the secretion of NO and IL-6 in differentiated macrophages. The findings suggested that RBAPS has the potential immunomodulatory and anti-tumor effects, giving it application prospect in the health and pharmaceutical fields.